The invention relates to an illumination unit for a device having a reflective liquid crystal display (LCD), comprising a flat waveguide above the LCD with a lower side facing the LCD and an upper side remote from the LCD, and a light source for coupling light into the flat waveguide.
Such an illumination unit is known from European patent application EP-A-0 778 484. The flat waveguide described in this document comprises two parts placed on each other. One part is anisotropic with respect to light and thus exhibits birefringence, whereas the other part is isotropic. The interface between the two parts is grating-shaped, formed by a one-dimensional grating. Color selection takes place by means of colored filters between the flat waveguide and the reflective parts of the reflective liquid crystal display. The refractive index of the isotropic part of the flat waveguide should be preferably equal to one of the two refractive indices of the anisotropic part of the flat waveguide.
It is a drawback of the known device that it is not easy to manufacture and comprises a plurality of elements which must be positioned with respect to one another.
It is an object of the invention to provide an illumination unit for a device having a reflective liquid crystal display, which unit has a simple structure and does not require selection of materials which must have equal refractive indices, be transparent, uncolored and positioned against one another in the form of a grating.
According to the invention, this object is achieved in that the light source is adapted to alternately emit light of a plurality of colors, which plurality may also be one, in that at least one side of the flat waveguide is provided with a diffraction grating, and in that the diffraction grating comprises at least as many areas tuned to different colors of light as the plurality of different colors of light emitted by the light source.
It is thereby achieved that a simple structure is possible for a light source emitting light of one color, without selection of transparent, uncolored materials having equal refractive indices and without the need for positioning parts against one another in the form of a grating. In addition, it is achieved that, for a light source emitting more colors of light, the color selection of the light takes place time-sequentially by means of the diffraction grating, so that only one color of light defined by the light source is each time transmitted through the flat waveguide towards the reflective liquid crystal display. The diffraction grating diffracts the light incident at an acute angle towards the reflective liquid crystal display, provided that this light has the correct wavelength and is incident at the correct angle so as to be able to contribute to the formation of the image. The light which is subsequently reflected by the reflective liquid crystal display towards the flat waveguide no longer encounters a grating on its path, which grating has such properties that reflection takes place on this grating. The light coming from the reflective liquid crystal display thus passes the grating in an unhindered manner.
The preferred embodiment of an illumination unit according to the invention is characterized in that the tuned areas are subdivided into small areas which are distributed on the relevant side of the flat waveguide.
In this way, it is ensured that the illumination of the reflective liquid crystal display is evenly distributed across the surface area of the reflective liquid crystal display so that all reflective elements of the reflective liquid crystal display are illuminated in a sufficiently uniform manner.
A further preferred embodiment of a unit according to the invention is characterized in that the grating period of a small area of the diffraction grating is of the order of the wavelength of the light to which it is tuned.
It is thereby achieved that a sharp selection of the color of the light is possible, which light is reflected by the diffraction grating exactly in the desired direction of the liquid crystal display, particularly the area which is tuned to the relevant color of light. It is true that all other colors of light are also scattered on the area which is tuned to the first-mentioned color, but this takes place in other directions, with the result that, after reflection on a reflective element of the reflective liquid crystal display, the light rays of these different colors have such a direction that they do not contribute to an image which is visible to a viewer. This is particularly important and advantageous in a situation in which the reflective liquid crystal display has small dimensions, of the order of a half to 1 inch, and the image must be viewed with an optical device. In such a situation, only those light rays that reach the relevant optical system are important.
A further preferred embodiment of a unit according to the invention is characterized in that the small areas for the different colors of light are evenly distributed on the relevant side of the flat waveguide.
It is thereby achieved that a uniform illumination of the reflective liquid crystal display is possible with every color of light emitted by the light source.
A further preferred embodiment of a unit according to the invention is characterized in that the total surface areas of the small areas are mutually inversely proportional to the intensities of the different colors of light emitted by the light source.
It is thereby achieved that, if the light source does not emit light of the same intensity for each color, the intensity of the image finally shows a good balance between the different colors of light constituting the image, so that a viewer perceives all colors of light as being emitted with the same intensity by the light source. It is also achieved that the selection of sources for the different colors of light is larger than in a situation in which the intensities of the different colors of light emitted by the light source must be accurately equal to one another.
Yet another preferred embodiment of a unit according to the invention is characterized in that the light from the light source is polarized.
It is thereby achieved that the effectiveness of the diffraction grating increases. Particularly in a situation in which the grating periods of the areas of the diffraction grating are of the order of the wavelength of the light to which the areas are tuned, polarization of the light coupled into the flat waveguide by the light source plays an important role. Polarization of the light emitted by the light source ensures that the ultimate image has a considerably higher contrast ratio than in the case where the light emitted by the light source is not polarized.
A further preferred embodiment of a unit according to the invention is characterized in that a polarizer is arranged proximate to the upper side of the flat waveguide.
It is thereby achieved that light polarized in a direction which does not correspond to the direction of polarization of the light emitted by the light source and thus has a contrast-decreasing effect, is obstructed, so that the contrast of the image is enhanced.
A further preferred embodiment of a unit according to the invention is characterized in that the polarizer is a reflective polarizer.
A reflective polarizer is a material which reflects incident light polarized in a first direction and transmits light polarized in a second direction perpendicular thereto. It is thereby achieved that the efficiency can be enhanced considerably in the case of a correct position of the reflective polarizer as regards the direction in which polarized light is reflected. In this context, the correct direction means that the reflective polarizer reflects light having the same direction of polarization as the polarized light emitted by the light source, and in the other case, polarized light is passed.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.